Amplifier circuit



March 9, 1937. Y l P, Q FARNHAM 2,072,945

AMPLIFIER CIRCUIT 5 Sheets-S-Yheet 2 4 P. o. FARNHAM AMPLIFIER CIRCUIT Filed March 19, 1951 Gia/'n rs. Eveque/:cy

March 9, 1937.

P. o. FARNHAM 2,072,945

March 9, 1937.

^ AMPLIFIER CIRCUIT Filed March 19, 1951 `5 lsheets-sheet 5 IIIII/Illlllllllli J? yaa,

March 9, 1937. P. Q FARNHAM 2,072,945

AMPLIFIER CIRCUIT Filed March 19, 1931 5 sheets-Sheet 4 Besana/Ice for ja/zdara @alfa/7" l Lower /rrequenc/es /yocyc/es of Resonance @j @@MJQM,

`Mamh 9, 1937.

AMPLIFIER CIRCUIT Filed March 19, 1931 5 Sheets-Sheet 5 I TC T6' V- T" TC i: LllllllllllhlllllllFl M T0 l; l LIIIIIIIIIIIIIIIIIIIIIIIIll'- E226 J f i' mm1 o P. o. FARNHAM 2,072,945

Patented Mar. 9, 1937 UNITED STATES AMPLIFIER 'CIRCUIT` Paul 0. Farnham, Boonton, 1.1, assignor,` by' mesne assignments, to Radio Corporation of America, New York, Delaware Application March 19,

30 Claims.

This invention relates tov amplifier circuits and particularly to methods of' and circuit arrangements for rendering the dynatron suitable for use in tuned high frequency amplifiers.

The more usual design requirements for a tuned amplier are ireedomV from oscillation and freedom from marked variation in gain as the amplifier is` tuned over ay range of frequencies. For some uses, for example in the caseofiradio receiversfwhere two orV more tunedr stages may be cascaded,. the requirements are comparatively rigid; asc the. stability, sensitivity and selectivity of? ther: receiver; must be maintainedv under all operating-adjustments to Whichv the receiver may i5 besubrected byi an unskilled operator.

Since they conditions Whichgivezrise to. oscillations,-. and;.whichwcause variations ingain and/or selectivity with changing adjustmentsv of the operating controls in thecase of the dynatron are quitedifferent;y from -thosewhich produce: disturbing,` eiects in4 the; case'of the` usual orV positive resistance tubes,k the known: methodsfof securing stability and satisfactoryr sensitivity and selectivity characteristics are not applicableY to the dynatrorr. Furthermore, it is well known that af-dvnatron will act as a satisfactory oscillator when a-tuned impedance, of appropriate design,

is. included;k irrthe: vplate circuit ofthe dynatrorr.

This. accepted useof. the dynatronr as va. generator oi oscillations,` as well as: the. peculiar problems encountered in attempting. to operate tliev dynatron irraaizuned-` amplifier have, so far as Iam aware, restricted fthe.use ofthe dynatron amplier to circuits. havinga non-reactive plate load',

s i...e. a plate load consisting of a positive resistance having avmagnitude. appreciably less than tnenegative plateresistance of the dynatron.

Obiects ciV the invention are to provide methodsz'of and circuit arrangements for employing 40 thedynatron inazamplifier stages of the type having.. a resonant"v output circuit. Further objects arefzto provide methods of-and circuitarrangements: forl employing dynatrons in tuned high frequency amplifiers. Further objects are. to

7 provide :methods of'iandlucircuit arrangements for preventing self-oscillation and for maintaining satisfactory'operating characteristics throughout the entire.. range of` operating adjustments of a dynatron ampli-fier having a tuned output circuit.

More specifically, objects-'of the invention are to provide, in adynatron amplier having'A a tuned output'4 circuit, methods of and circuit elements iorautomatically maintaining an approximately constant ratio. between the magnitude of the D plate: resistanceY and the plate load, and/orfor N. Y., a corporation ofV 1931, Serial No. 523,859

automatically preventingy that ratio from reaching or exceeding unity.

These and other objects of the invention will be apparent from the following specification, When taken with theraccompanying drawings, in Which Fig. l is a curve sheet-illustrating, for different control grid potentials; the relation between plate current and-plate voltage in a dynatron of the tetrode or screen grid type, n

Fig. 2 is a circuit dia-gram of a single stage dynatronamplifier, tunable over a range of frequencies, Y

Fig.. 3 is a curve. sheet illustrating theV relation between stage gain and frequency,- for different controlgrid bias voltages, in an amplier such as shown inFig. 2,

Figa. 4 is a circuit'diagram of cascadeddynatron amplifier stages embodyingthe invention',v

Fig. 5Y is a-curve sheetA illustrating;V the: selectivity, for the same gain, obtained with adynatron amplifier embodying the invention, and with a known amplifier of the'positive plate resistance type;

Fig. 6 is a curve sheet illustratingthe selectivity of a dynatron` amplifier stage for. various amplification ratios-andafat onevalue of circuit tuning, and

Figs. 7 to 11 inclusive are circuit diagrams illustrating other embodiments of the invention.

The: several curvesa-d off Fig'. I are typical curves showing` the relationship existing-between plate current and plateV voltageirin'! a` dynatron of. the tetrode type for diierent' values of control grid bias Ec, the characteristic curvefa, corresponding to a large negative bias.: while curve d, corresponds? toa relatively'Y low-negative bias.

through .point EB, and the' several characteristic curves which' correspond to bias voltages a, b; c and d;

Further, the value' off th'ejn'egative plate resistance at a given biasr` voltageri's represented by the slope of the curve at the point oi' inter- For the plate supplyy voltage EB, theV section. For example, the plate resistance rp of the tube for a bias voltage corresponding to curve b and plate voltage EB is equal to tan 01, or

where cgis the alternating voltage between control grid and cathode, ep is'the alternating voltage between plate and cathode, and

for constant plate current with no load between plate and cathode. j

Since the load impedance is positive and the plate resistance is negative under all conditions now underconsideration, it is evident that the load impedance RB is always greater than the negative quantity rp.- For convenience of discussion, however, the algebraic sign of these quantities may be neglected in the following explanation by comparing the magnitudes or numerical Values of these impedances.

When the plate load is less than the plate resistance, RB 1'p,kthe tube and associated circuits will operate satisfactorily as an amplifier.

Reverting to Fig. 1, Vit will be seen that this condition obtains at operating points e and f. When the external impedance and plate resistance are equal, RB=7'p, the amplification as given by equation (1) becomes infinite since the term reduces to zero, and oscillation of the system is impending. This condition obtains at operating point g, since the slope of curve c at this point is equal to the slope of the line 1' through that point. When the load is of greater magnitude than the plate resistance, the systemwill oscillate. This condition obtains atpoint h on curve d, i. e., at negative grid bias voltages less than that corresponding to curve c.

It is therefore apparent that a dynatron amplier having a tuned plate load impedance will oscillate at any frequency for which the plate load is a pure resistance, RB, that is greater than the plate resistance rp. This oscillating condition may be reached either by adjusting the control grid bias towards less negative values or by increasing the plate load.

An amplifier system adapted to the selective amplification of-signals over a band of frequencies should retain certainY features of performance throughoutA its entire range of adjustment. The amplifier should not oscillate at any adjustment of the amplification control, nor should it oscillate at any frequency to which the system may be tuned.

In accordance with the present invention, the tendency to oscillate as an electrode voltage is adjusted to increase the amplification is overcome by a simultaneous and automatic adjustment of another electrode voltage. One method of obtaining this action, when the amplification rate is controlled by varying the control grid bias, is by introducing into the plate circuit a resistance R of such value, tan 0, that the plate potential varies with grid bias to avoid conditions favorable or oscillation at any gain control adjustment of grid bias. Referring again to Fig. 1, a consideration of the operating characteristics of the dynatron will show that the operating points, for different grid bias potentials when a direct current resistance R=tan 6 is included in the plate circuit, may be determined graphically by drawing a line through point EB and at an angle 0 to the ordinate through that point. For control grid bias voltages corresponding to curves a to d,'the points vof intersection z to l, respectively, indicate the operating points, and it is to be noted that, within the region in which the plate-cathode resistance has a negative resistance, the insertion of a positive resistance in the plate circuit has the effect of impressing upon the plate a direct current voltage which is greater than the plate supply voltage EB.

By appropriate by-passing of the resistance R for carrier frequencies, the alternating current impedance of the plate load is not affected by the introduction of resistance R and the graphical analysis of the problem of insuring stability of operation may be completed by drawing through points z', h, etc., the lines r to indicate the magnitude of the resistance of the resonant plate load EB. an inspection of Fig. l will show that stable operation has been insured by the insertion of the plate resistance R since, for all values of gain control up to a control grid bias corresponding to curve d, the resonant plate load RB is less than the plate impedance rp. Ap-

plying the test for oscillation, it is seen that for each indicated operating point, the slope of the line 1' is less than the slope of the plate current characteristic at that point. By inspection, it will be observed that these slopes approach most closely at the operating point lc, which indicates that maximum amplification obtains at a control grid bias indicated by curve c.

Reverting to the question of freedom from oscillation as the output circuit is tuned over a band of frequencies, it has long been recognized thatthe impedance, at resonance, of a tuned circuit varies with the frequency to which the circuit is tuned. If the plate load impedance RB should vary widely with the tuning of the output circuit, these variations will -be greatly exaggerated in the gain-frequency.characteristic of the dynatron stage for a constant value of rp (i. e. a constant control grid bias as represented by the curves of Fig. l) as may be seen from an inspection of Equation (l), particularly when RB and rp approach equality. Unless due precautions are taken, the tuning of the output circuit may so alter the magnitude ofthe load impedance that oscillatory conditions are established. Optimum conditions'so far as concerns stability of operation with tuning, may be obtained by maintaining a constant ratio between the magnitudes of the external plate load and. the plate-cathode resistance. This condition may be realized, either completely or to an extent sufcient to prevent oscillation, by maintaining the plate load substantially invariant with tuning for a fixed plate resistance, or by varying thek plate resistance as a substantially linear function of a varying plate load. In accordance with the first method, the plate circuit includes a compensating or corrective network, in addition to the tuned output circuit, which maintains the load impedance, as viewed from the plate and cathode terminals, substantially independent of the tuning of the output circuit.

One satisfactory embodiment of the invention for securing operation which conforms to the requirementspredicated upon Fig. 1 is illustrated diagrammatically in Fig. 2. As indicated by the above discussion, the vacuum tube is of the tetrode or screen grid type having an input circuit which may be a resonant circuit LC connected between control grid G1 and an equipotential cathode K, which is heated to emitting ternperature by a heater H. The source of current supply for impressing voltages on the several tube electrodes is indicated diagrammatically as a battery B, but it is obvious that any suitable sour-ce, such as the customary power supply devices, may be employed. As indicated, a negative bias Ec is applied to the control grid, and the screen grid G2 is connected to a point on the current source at which the voltage En is greater than the supply source voltage EB which determines the plate voltage.

The tuned output circuit LzCz is not connected directly across the plate P and cathode K, but is coupled to the plate through a condenser Cm. The direct current potential from supply source B is impressed upon plate P through a shunt circuit which includes a choke L1 and resistances R1 and R2 which have a total resistance equal to that of resistance R:tan 0 of the above discussion of Fig. 1. Condensers C are provided to bypass the plate resistance R2, the screen grid and cathode leads for carrier frequencies.

Many factors such as the type of tube, the maX- imum gain desired and the frequency range, have a bearing upon the choice of the Values of the several circuit elements, but, as illustrative of one practical arrangement, the following values are cited as suitable when a tube of the commercial UY 224 type is employed:

EB volts Ec :from -10 to -14 volts En Volts R1 :2500 ohms R2 :15,000 ohms LC :tunable from 550 to 1500 kilocycles L1 :2.75 millihenries Cm :about 15 micromicrofarads L2 :approximately 220 microhenres C2 :adjustable from 40 to 440 micromicrofarads C :approximately 0.5 microfarad The inductance L1, resistance R1 and associated condenser C form one path from plate to cathode, with a second path through the coupling condenser Cm and parallel tuned circuit L2C2. The resistance R1 is necessary to prevent oscillation at a frequency determined substantially by the choke L1 in shunt with Cm and the inherent platecathode capacity of the tube. It is also useful in holding down the value of the load impedance at the lower end of the frequency band. The combined resistances R1 and R2 act to increase the plate voltage, for a constant plate supply voltage EB, as the control grid bias Ec is reduced to increase the stage gain. The curves A, B and C of Fig. 3l indicate the observed relation between gain and frequency for three different adjustments of the control grid bias E@ in the dynatron circuit shown. in Fig. 2. Since' the gain isy approximately independent of. frequency, it is apparent from a consideration of Equation (1), that the value RB is substantially independent. of frequency. The'l observed performance of this dynatron stage therefore demonstrates that the vparallel circuit through which the direct current potential is applied to the plate' acts as a comkilocycles Two or more stages of dynatron amplification may be. readily'cascaded in accordance' with the present invention. One convenientv circuitarrangement is illustrated in Fig. 4, in which the circuit elements of; each stage ma-y be, and except as here specied are preferably, substantially identical with the corresponding elements of Fig. 2. The tuned input circuit LC of the rst dynatron V1 is coupled through the condenser C0 to the collecting structure which. is diagrammatically shown as an antenna system. The general arrangement of the stage elements is as shown in Fig. 2 and the various elements are identified by the same reference characters. novel feature peculiar to the cascading of two dynatron stages resides in the provision of a resistance R which is shunted across the low potential terminals of the resistances R1 of. the cascaded stages. An adjustable tap I0 on this resistance R is connected to the direct current supply source B to impress a direct current voltage upon the plates P of the cascaded tubes V1 and V2. The significance of this arrangement is that the total resistive impedance in the plate `circuits of the respective dynatrons may be so adjusted that the tube characteristics are equalized toy give maximum gain at the same value of the common control grid` bias Ec.

In both of the described embodiments of the 1 invention the frequency uniformity of gain is accompanied by a further operative improvement which is indicated by the selectivity curves of Figs. 5 and'. This improvement in selectivity may be considered as resulting from a decrease in the net positive resistance of the tube and output circuit as a whole, and becomes greater as the load impedance approaches the magnitude of the plate resistance. Hence when the stage gain is increased by using less control gridv bias Ec, and

n rv

stage may be made, in accordance with this in-i:

The.

Ivention, substantially more uniform over the tuning range than in the case of the positive resistance tube and tuned circuit.

The solid line curves 600, 1000 and 1330, of Fig. indicate the selectivity of the dynatron circuit of Fig. 2 for signals of these three frequencies and for the same amplication. The dotted line curves identied by the same numbers indicate the selectivity of a typical radio frequency amplier stage of the positive plate resistance type for the same amplification. It will be noted that the selectivity of the dynatron amplifier is superior to that of the positive resistance amplifier, and that there is less variation with frequency in the case o-f the dynatron.

The curves of Fig. 6 show the relation between amplification and selectivity at a single frequency of 1000 kilocycles. The lower curve, identified by reference numeral 1000, shows the selectivity for a stage input, En, of. 1000 microvolts, the curves 100 and 10 being corresponding selectivity curves for input voltages of. 100 and 10 micro'- volts, respectively, with the amplifier gain increased to maintain a constant output level. It will be noted that the selectivity increases rapidly as he stage gain is increased for decreasing signal strength.

Other circuit arrangements may be employed for maintaining the desired ratio between plate load impedance and plate resistance and some appropriate designs are shown in Figs. 7 to 11 inclusive. The circuit elements which are, or may be, substantially identical with those shown in Fig. 2 are identified by the same reference Characters but will not be described in detail.

As shown in Fig. 7, the tuned output circuit is not a. simple parallel tuned circuit, but includes an adjustable resistance R in series with the inductance L2, the adjustable tap of. the resistance being mechanically connected to the adjustable element of the tuning condenser C2. The rate of variation of the effective value of R with tuning is so chosen that the impedance at resonance,

RB, remains approximately constant over the tuning range.

In the arrangement shown in Fig. 8 a corrective network comprising resistance R in series with a parallel LiCi circuit is in shunt with the tuned output circuit L2C2. The general operation of the circuit is similar to that of Fig. 2, and, by appropriate choice of the values of the elements of the shunt circuit, the impedance of. the composite output circuit, as viewed from the plate and cathode terminals, may be maintained substantially independent of frequency.

The circuit of Fig. 9 includes a resistance Rd between the screen grid G2 and the current source B. The effect of resistance Rd is to alter the shape of the current-voltage curves as the control grid bias Ec is adjusted to control amplication, due to the automatic change in screen grid potential obtained by the voltage drop across Rd. By introducing suicient resistance in the screen grid circuit, the screen grid voltage will be reduced at such a rate that the plate resistance rp is greater than the plate load for all values of control grid bias.

The circuit illustrated in Fig. 10 provides for an automatic change in the magnitude of the plate resistance rp with the changing value of the load impedance RB as the circuit L2C2 is tuned over the band of frequencies. The control grid bias is determined by the cathode bias resistor R and the effective magnitude of this resistor is varied automatically with tuning by connecting theadjustable tap I of the resistor with the adjustable element of condenser C2, as indicated by broken line I2. The resistance R may be varied wtih frequency at such rate that remains substantially constant, thus ensuring substantially uniform gain at all frequencies, or the ratio may vary with frequency provided rp is never less than RB.

In the circuit arrangement illustrated in Fig. 11, the control grid bias Ec and plate voltage EB are varied simultaneously to ensure that the plate load will not be greater than the plate resistance for any operating adjustment of the amplifier. As indicated diagrammatically by the lever I3 and broken lines I4, the cathode and plate circuit taps to the supply source B are mechanically connected to increase the plate supply voltage EB as the bias voltage Ec is reduced to increase the stage gain. Referring again to Fig. 1, it will be apparent that plate supply Voltage EB may be increased at such rate as to effect operation at points i, 7', 7c and Z, as the control grid bias is successively decreased to Values corresponding to curves a to d, respectively.

ToY those familiar with the design and operation of amplifier circuits, it will be apparent that the described methods for securing stability of operation, and/or a predetermined gain-frequency characteristic, may be carried out with other circuit arrangements. While a substantially uniform gain for all frequencies is usually desirable, it is obvious that the invention provides a method of securing other gain-frequency characteristics if such are desired for special uses.

In the following claims, the term plate load impedance refers to the impedance, at resonance, of the output network as viewed from the plate and cathode terminals of the tube; and the term plate impedance is used in its customary sense to identify the internal impedance, rp, of the tube.

I claim:

1. In a dynatron amplifier, the combination with an electron tube and means for applying to the elements thereof potentials effective to give the same a negative plate resistance, of a tuned output circuit connected to the plate electrode of said tube, and means rendering the ratio of plate load impedance to plate resistance substantially independent of the frequency to which said output circuit may be tuned.

2. The invention as set forth in claim l, wherein said means comprises circuit elements cooperating with said tuned output circuit to render the plate load impedance substantially independent of the frequency adjustment of said tuned output circuit.

3. The invention as set forth in claim 1, wherein said means comprises circuit elements `constituting a path between plate and cathode rent potential is more positive than that of the` rtube cathode.

5. An amplifier circuit comprising a tube and means applying to the electrodes thereof direct current potentials effective to give the plate impedance a negative characteristic, and an output network for said tube, said output network Yincluding impedances Aproviding two `parallel pathslbetween V plate and cathode, one path comprising aninductance and resistance serially connected between the plate-and the source ofplate current supply, Vthe s-econd path comprising a tuned output circuit coupled to said plate through a capacity.

6. The linvention as defined in Vclaim 5, wherein a condenser is connected between a point n said resistance and .ground to by-pass v a portion of-said resistance and the plate current supply l.for .alternating currents of frequencies within the tuning lrange of saidputputcircuit.

7. `In a dynatron ampliiier, the combination with an electron tube and means energizing said tube for dynatron operation, of atuned output circuit whose impedance at resonance varies with the frequency to A.which said circuit is tuned, and means other than said tuned circuit Vfor automatically varying the plate impedance of the tube as the output circuit is tuned over its frequency range.

.8. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation, of atuned output circuit Whose impedance at resonance varies with the frequency to which said circuit is tuned, means for automatically varying the plate `impedanceof the tubeas the output circuitis tuned over its frequency :range and at such rate that the ratio of plate load impedance to plate -impedance remains approximately constant.

9. In :a dynatron amplifier, fthe combination with an electron tube and means .energizing said tube for dynatron operation, .of a'tuned output .circuit whose .impedance at resonance varies Vwiththe 'frequency'to which said circuit is tuned,.and means for automatically Varying the direct current potential applied :to one of the tube electrodes as the output circuit .is tuned over its frequency range.

10. The invention as set forth in claim 9, wherein Asaid means includes an adjustable circuit element for varying the control grid bias, and a mechanical connection between saidadjustable circuit element and the variable element of said tuned circuit.

ll. In an amplifier, a pair of-cascaded stages comprising a pair of electron'tubes energized to l operate-as dynatrons, a tuned output circuit I'for each tube, means for-effecting Ya simultaneous adjustment of 'the'gain of each stage, and ladjustable means determining the relative `rates at which the Ygain Yofthe stages is affected by the action of said iirst means.

12. In an amplifier, a pair of cascaded stages comprising two electron tubes energized to operate as dynatrons, a tuned output circuit for each tube, and means associated with each stage for preventing oscillation thereof, said means including circuit elements adjustable to determine the relative gain of the individual stages.

13. A cascaded dynatron amplifier comprising a pair of electron tubes energized to operate as dynatrons, a tuned output circuit for each of said tubes, a connection from the plate of each tube to a source of plate current supply, said connections being in parallel with the tuned output circuit of the respective tubes and including inductance and resistance, means for adjusting the amplification of said tubes, and means for varying the resistance included in one of said connections, thereby to provideya,second -amplicai tion control forthe tube associated with saidad- `iustable resistance. y K

14. :Aicascaded dynatron amplifier, comprising the combination with ytwo ytubes energized to operate as dynatrons, a-tuned circuitfcoupling the two gtubes, 2a tuned output-circuit for the 1second tube, of .meansforpreventing oscillation ,of either tube, said means comprising-'acircuit coupled between theyplates of ,the said tubes and comprising an inductance connected .-to-.theplata-of each tube-'and three resistancesseriallyY connectedsbetween thesaidiinductances,@anadjustable tap on the lcenter :resistance :of fsaid vthree `resistances andconnected` to lthe -sourceof plate current supply Aand.- capacitiesA -by-passing'said center resistamatori-alternating curr-.ents of; frequencies withinthe rtuning, range ,offfsaid 1 tuned circuits.

15. In ,combinationr a 4vdynatron amplifier tube network provided ,with a -high .frequency input circuit and a load circuit coupled totheamplier anode, .-an vadjustable :circuit element `.connected to one rvof ythe .electrode circuits of said vamplifier, theratio ...of `load impedance to tube impedance varying .with adjustment of:said adjustable element, and means forimaintaining said ratio-substantially iless -than unity .f-as .said element is adjusted. j

16. yIn combination, a dynatron Aamplifier tube network provided with a high :frequency input circuit and a load circuitccupled1 to thez amplifier anode, `an adjustable circuit element .connected toone of :the electrode circuits .offsaid amplifier, the ratio Aof load impedance :to tube impedance varying with adjustment of:saidadjustable.element, and means v for #maintaining-saidratio-substantially -less -,than unity 'and :substantially constant as fsaid element is-adjusted. l

17. .In combination, sa dynatron yamplifier .tube network provided with ,a high .frequencyinput circuit and a load circuit coupled totheamplier anode, an adjustable circuit element connected-to one ofthe electrode circuits .of saidampliiieLthe ratio ,of -load impedanceltoft'ube impedance fvarying with adjustment of said adjustableelement, and adjustable :means ,simultaneously operative with said adjustablefelementfor maintaining Asaid ratio substantially less than unity as said element is adjusted.

18..-A lmethod of y'controlling the 4.gain of v.a dynatron-:amplifier vwhich `consists in varyinggin an increasinglyinegative potential direction .the controlI grid :bias thereof 'to :reduce the vamplifier gain, andsimultaneously.varyingvthezpotential'on another yelectrode, ,in .an increasingly `positive sensefto maintain the, internal `plate ,resistanceof the amplier l:more:nearly )independent of x,the variation of said grid bias.

19. In combination with a screen grid tube provided with input and output circuits and means for applying potentials to the tube electrodes such that the tube and circuits provide a dynatron network, a volume control element connected to the control grid of the tube, and an additional element, responsive to adjustment of said control element, for varying the potential of an electrode other than said control grid.

20. In combination with a screen grid tube provided with input and output circuits and means for applying potentials to the tube electrodes such that the tube and circuits provide a dynatron network, a volume control element connected to the control grid of the tube, and an additional element, responsive to adjustment of said control element, for varying the potential of an electrcde other` than said control grid in a direction to maintain the network free of any tendency to produce oscillations.

21. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning.

22. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintainingthe total impedance of said output circuit substantially constant during tuning, said means comprising an aperiodic reactive network.

23. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising an aperiodic reactive network in series in said output circuit.

24. In a dynatron amplifier, the combination With an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising a network whose effective resistance varies with Variation of tuning.

25. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising an aperiodic network whose effective resistance increases with decreasing frequency.

26. In a dynatron amplifier, the combination with an electron tube and means energizing said tube for dynatron operation of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and means in said output circuit for maintaining the total impedance of said output circuit substantially constant during tuning, said means comprising a network consisting of a resistor shunted by a capacity whose effective resistance increases with decreasing frequency whereby the total impedance of said output circuit is maintained substantially constant over the tuning range of said circuit.

27. In a dynatron amplifier, the combination with an election discharge tube and means energizing said tube for dynatron operation, of a tuned output circuit whose impedance at resonance varies with the frequency to which said circuit is tuned, and an aperiodic reactive network having one endthereof connected to the anode of said tube and the other end thereof to the said tuned circuit for maintaining the effective impedance of said circuit substantially constant as the output circuit is tuned over its frequency range.

28. A method of controlling the gain of a screen grid amplifier which consists in varying the control grid bias thereof in an increasingly negative potential direction to reduce the amplier gain, and simultaneously varying the potential on another electrode in an increasingly -positive sense to maintain the internal plate re sistance of the amplifier more nearly independent of the variation of said grid bias.

291. A method of controlling the gain .of a screen grid amplifier which consists in varying the control grid bias thereof in an increasingly negative potential direction to reduce the amplifier gain, and simultaneously varying the potential on the plate electrode in an increasingly positive sense to maintain the internal plate resistance of the amplifier more nearly independent of the Variation of said grid bias.

30. In combination with a signal frequency amplifier including a tube provided with at least a cathode, signal input grid, positive screen grid and positive plate, a tunable signal input circuit connected between the input grid and cathode, a tunable signal output circuit coupled to the plate and cathode of said tube, means for varying the potential difference between said input grid and cathode over a desired range of values to adjust the gain of the tube, and a network electrically associated with the plate circuit of said tube having its constants so chosen that the amplifier load impedance approaches the magnitude of the plate circuit resistance as the tube gain is increased whereby the selectivity of the signal amplifier is a-maximum when the gain varying means is adjusted to secure maximum amplifier gain.

PAUL O. FARNHAM. 

